I used the screws that looked closets to a perfect fit. Not the longest but the second to the longest. The instructions leave much to be desired. I saw on a video where some poor guy lost the wings during a turn and it made me think about the wing struts. You notice the instructions say nothing about gluing the wing struts? I suggest you had better if you haven't. That is why that guy lost his and I took mine up for its maiden fly last night and it came back missing a wing strut. Now I've got to find and place to buy a new wing strut for mine.